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In-Reply-To: <868F0874-70E8-4416-B39B-DA74C9D76A40@fb.com>
From:   Yu Zhao <yuzhao@google.com>
Date:   Wed, 10 Aug 2022 15:56:06 -0600
Message-ID: <CAOUHufas1o1Ez9EDSjjxsO-w6Ph6Km8R8yOX+1_c=4LvG6DEWw@mail.gmail.com>
Subject: Re: [PATCH v3] mm: add thp_utilization metrics to /proc/thp_utilization
To:     "Alex Zhu (Kernel)" <alexlzhu@fb.com>
Cc:     Yang Shi <shy828301@gmail.com>, Rik van Riel <riel@fb.com>,
        Kernel Team <Kernel-team@fb.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "willy@infradead.org" <willy@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Ning Zhang <ningzhang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>
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On Wed, Aug 10, 2022 at 3:39 PM Alex Zhu (Kernel) <alexlzhu@fb.com> wrote:
>
>
>
> > On Aug 10, 2022, at 10:54 AM, Yu Zhao <yuzhao@google.com> wrote:
> >
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
> >
> > |-------------------------------------------------------------------!
> >
> > On Wed, Aug 10, 2022 at 11:15 AM Alex Zhu (Kernel) <alexlzhu@fb.com> wr=
ote:
> >>
> >>
> >>> On Aug 10, 2022, at 10:07 AM, Yang Shi <shy828301@gmail.com> wrote:
> >>>
> >>> On Tue, Aug 9, 2022 at 4:36 PM Yu Zhao <yuzhao@google.com> wrote:
> >>>>
> >>>> On Tue, Aug 9, 2022 at 11:16 AM Alex Zhu (Kernel) <alexlzhu@fb.com> =
wrote:
> >>>>>
> >>>>>
> >>>>>> OK, it is hard to tell what it looks like now. But the THPs on the
> >>>>>> deferred split list may be on the "low utilization split" list too=
?
> >>>>>> IIUC the major difference is to replace zero-filled subpage to spe=
cial
> >>>>>> zero page, so you implemented another THP split function to handle=
 it?
> >>>>>>
> >>>>>> Anyway the code should answer the most questions.
> >>>>>
> >>>>> They can indeed end up on both lists. This did have to be handled w=
hen
> >>>>> implementing the shrinker.
> >>>>>
> >>>>> We free the zero filled subpages, while modifying the existing spli=
t_huge_page()
> >>>>> function. Will follow up that change in another patch.
> >>>>
> >>>> FYI. This series does it:
> >>>>
> >>>> https://lore.kernel.org/r/20210731063938.1391602-1-yuzhao@google.com=
/
> >>>>
> >>>> And this one:
> >>>>
> >>>> https://lore.kernel.org/r/1635422215-99394-1-git-send-email-ningzhan=
g@linux.alibaba.com/
> >>>
> >>> Thanks, Yu. I totally forgot about these series. It is time to refres=
h
> >>> my memory.
> >>
> >> I looked through these patches yesterday. There are indeed parts that =
are very similar, but the approach
> >> taken seems overly complicated compared to what I have written. What=
=E2=80=99s the status of work on this since last year?
> >
> > Overly complicated... which patches and how?
> >
> > At a minimum, you'd need 1 & 3 from the first series and this patch:
> >
> > https://lore.kernel.org/r/20220608141432.23258-1-linmiaohe@huawei.com/
>
> The changes from the previous patches implement freeing of THPs as part o=
f memcgroup and reclaim. Zero tail pages are disposed of via
> lruvec as part of reclaim.

Which series are you talking about? I listed two series and they are
very different on the code level.

> Our approach is a thp utilization worker thread scanning through physical=
 memory adding under utilized THPs to a shrinker that calls split_huge_page=
(). We free zero tail pages within split_huge_page(). Reclaim will trigger =
the shrinker.
>
> There is some overlap between the implementations, in particular creating=
 a linked list in the third tail page and methods to check for zero pages.
> (I believe the previous patches have a cleaner method for identifying zer=
o pages). However, looking through the code I do believe our approach is si=
mpler.
>
> We chose to free within split_huge_page()

The 2nd patch from the first series does exactly this.

> but it=E2=80=99s worth discussing whether to free zero pages immediately =
or to add to lruvec to free eventually.

And that patch can be omitted if the third link (a single patch, not a
series) is used, which makes the workflow "add to lruvec to free
eventually".

> I believe the split_huge_page() changes could be valuable by as a patch b=
y itself though. Will send that out shortly.